Interactive television and data transmission system

ABSTRACT

A spread spectrum system provides bidirectional digital communication on a vacant television (TV) channel for simultaneous use by more than 75,000 subscribers using time and frequency division multiplex signals locked to horizontal and vertical sync pulses of an adjacent channel Host TV station. The system, whose operation is analogous to a radar system, comprises: (1) the Host TV station to send down-link sync and data pulses to subscribers during the horizontal blanking interval (HBI), (2) subscriber &#34;transponders&#34; which detect those signals and transmits up-link &#34;echo&#34; data pulses only during the HBI to eliminate interference to TV viewers, and (3) a central receiver which also uses the host TV sync pulses to trigger range gates to detect the up-link data pulses. In a preferred embodiment the central receiver employs directional antennas to determine direction to transponders and to define angular sectors partitioning the service area into pie-link &#34;cells&#34; which permit frequency re-use in non-contiguous sectors (like cellular radio). The system thus operates like a radar to measure elapsed time between receipt of TV sync pulses and receipt of transponder response pulses and measures bearing to transponders to thereby determine the location of fixed or mobile subscribers as well as provide data links to them. Transponders may share user&#39;s existing TV antenna or may operate on cable TV and could be packaged and &#34;RF modems&#34; for personal computers, as transceivers for mobile or portable use, or they may be integrated with a TV receiver to provide &#34;interactive television&#34;.

BACKGROUND OF THE INVENTION

This invention relates to a new system referred to as "T-NET" whichprovides bidirectional communication of digital information to aplurality of fixed or mobile subscribers on a vacant TV channel adjacentto, and cooperating with an existing "Host" television (TV) station. Thehorizontal and vertical sync pulses of the host TV signal are used as awide-area clock to coordinate time and frequency division multiplexingof subscriber transponders and to trigger up-link responses from themonly during the horizontal blanking interval (HBI) to preventinterference to television viewers Down-link signals to subscribers arealso sent within the HBI. In a preferred embodiment the typicalsubscriber-to-central receiver data rate is 300 or 1200 baud and thatsignal's spectrum is "spread" into subchannels 187.5 KHz wide by virtueof modulating it on a stream of 5 microsecond pulses. Thirty-two ofthese subchannels fit in a standard 6 MHz TV channel More than 300transponders can operate simultaneously on each subchannel. The samesubchannels may be used for up-link and down-link communications, evensimultaneously on the same subchannel.

Means to multiplex information to TV receivers on an existing TV signalduring its horizontal or vertical blanking interval are in use or havebeen contemplated (e.g. present day "Teletext"). However, the advantageof using TV horizontal and/or vertical sync pulses to synchronize bothdown-link and up-link radio signals, on the same or an adjacent TVchannel, so they effectively exist only within TV horizontal or verticalblanking intervals, thus are invisible to television viewers, and forthe further purpose of enabling time and frequency division multiplexingof many signals, has not heretofore been discovered. The presentinvention teaches that technology.

A major portion of the U.S. radio spectrum has been allocated tobroadcast services and more specifically to television. A substantialnumber of television channels are unused in most cities because ofphysical limitations caused by inadequate television receiverselectivity. As a consequence of this at least one vacant channel existsbetween assigned television stations and those channels have heretoforebeen unusable. As a practical matter, intermodulation interference andother considerations further limit the number of usable televisionchannels so that substantially less than half the allocated TV channelsare in use in a given area. Unusable channels are sometimes referred toas "taboo" frequencies. A principal object of the instant invention isto make practical use of this presently unusable spectrum.

Described another way, typical television receivers, particularly whenoperating at UHF frequencies, have relatively poor frequency selectivityconsequently radio transmission in channels adjacent to a TV signal isprohibited because it would cause unacceptable interference. Forexample, even a low power conventional radio device which transmits onewatt could easily cause unacceptable interference to adjacent channeltelevision viewers who live within a radius of several city blockssurrounding it because its power would overwhelm the TV signal. Clearly,thousands of such conventional transmitters deployed throughout a cityfor the uses contemplated here would generate unacceptable interference.

Since television broadcast channels are by government regulationallocated to "mass media" use, it is implicit that such channels are notintended for low capacity private radio communications such asassociated with point-to-point or land mobile radio applications.Consequently applications for the aforementioned vacant TV channels, ifthey could be used at all, would be expected to benefit the public enmasse as contemplated in the instant invention for such uses as futurehome information systems, interactive television, remote shopping,banking electronic mail, reservations means, security alarmcommunications, and the like.

Ease of installation and simplicity of operation are importantconsiderations for mass applications. Thus sharing of the user'sexisting television antenna as taught here is a important feature.Integrating this invention with a television receiver to provideinteractive TV controllable from remote hand-held devices comparable tothose used today for remote TV channel switching are other featurestaught in this specification.

An object of the present invention is to provide means to accuratelypartition subscribers into geographic "radio cells" within whichspecific subscriber transponder subchannels may be assigned and isolatedfrom transponders in other cells. This permits re-use of subchannelfrequencies in non-contiguous cells to significantly expand the numberof users that can operate on one previously vacant TV channel in a givencity. These desirable frequency re-use features are commonly identifiedtoday with "Cellular Radio".

The instant invention is also applicable to two-way cable TV systems(CATV) to provide improved isolation of up-link and down-link signalscompared to existing methods.

A further object of the present invention is to provide improved meansfor locating and tracking the position of mobile or portable subscribertransponders to provide economical services sometimes referred to asautomatic vehicle location or automatic vehicle monitoring (AVM).

Automatic "hand-off" of present day cellular radio telephone subscribersas they move from cell-to-cell in a city is a problem because it isbased on signal amplitude measurements and these vary widely atdifferent places and at different times. An independent means such asT-NET to locate subscribers can form the basis for an alternativehand-off method which could minimize or solve the existing problem andthis constitutes another T-NET application.

Yet another application of the invention is for so-called "videoconferencing" which usually comprises a dedicated TV network connectinga central office with many remote offices for such applications asover-the-air teaching, presentations by management, or even TVmonitoring of banks or other businesses for security alarm purposes.Such T-NET applications would employ the down-link to send pictures(video) and the up-link could either be digital or digitized"slow-voice", all multiplexed simultaneously with the existing TVprogram.

It is clear that simultaneous synchronization of the T-NET system withseveral TV stations in a city as contemplated by the inventor could be aproblem. Thus a further object of the invention is to teach an operatingmeans wherein the horizontal sync pulses of several co-locatedtelevision transmitters are locked together in time so that subscribertransponders working in cooperation with one or several such stationswill always transmit within the horizontal blanking interval (HBI) ofall the television signals simultaneously, thus eliminating inteferenceto viewers of all of them. TV transmitter co-location is a practice inmany large cities (e.g. Los Angeles and New York) to establish a commonantenna direction for all TV viewers.

Alternatively, it is taught that if T-NET subscribers are located in aboundary service area between television stations not co-located (e.g.between TV transmitters in adjacent cities), then those subscribertransponders can be programmed to transmit only during the verticalblanking interval (VBI, which is much longer in time duration than theHBI) and thus will not interfere with TV viewers of either city,provided those television stations are synchronized to cause theirvertical blanking intervals to overlap as taught in this invention.

Two new and improved methods are also taught for sending digitalinformation to subscribers (down-link) by either: (1) co-channelmodulation of the Host TV signal in a non-interferring manner or (2)modulating new "out-of-channel" subcarrier sidebands in adjacent upperor lower (or both) TV channels.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a bidirectional radio communication system foruse on presently vacant TV channels in cooperation with a hosttelevision transmitter. In one embodiment the host provides down-linkdigital signals to a plurality of subscriber transponders using theimproved methods herein set forth. Subscriber transponder devices detectthese signals and transmit carefully synchronized up-link digitalsignals to central receiving sites which are preferably located alongthe path between subscribers and the host television transmitter. Theinventor calls the system "T-NET".0

A network control center (NCC) interconnects the host televisiontransmitter and central receivers with Information Providers usingconventional trunk-line paths so as to furnish the Information Providerswith means to communicate with their subscribers, or to provide virtualcircuits for subscribers to communicate with each other. The InformationProviders may be organizations such as banks, retail stores, vehicledispatchers, Data Banks, entertainment sources such as pay TV, and thelike.

The horizontal and vertical synchronizing pulses normally transmitted bythe host television transmitter are employed in the invention as aclocking mechanism to coordinate time and frequency divisionmultiplexing of subscriber receiver/transmitter devices (herein calledtransponders). Subscriber transponders are triggered by the host TVsignal horizontal sync (hereinafter called "H-sync") pulses so that theytransmit only during the horizontal blanking interval (HBI) or verticalblanking interval (VBI). The HBI is typically eleven microseconds induration and viewers living within a radius of about one milesurrounding a transponder are simultaneously blanked out during thistime period. Consequently they would not see the transponder's signal,thus will not be interfered by it, providing its transmission durationis on the order of a few microseconds

The T-NET system is most easily described by comparing its operation toa radar system. The Host TV H-sync pulses are analogous to the outgoingradar pulses and these trigger transponder reply pulses ("echos"). Thereply-echos, each comprising one bit of information, are received at acentral receiver after a transit delay and that delay is a measure ofthe distance to the subscriber. In the United States the TV H-sync pulserecurrence frequency (called PRF in radar) is 15,734 Hz and provides anunambiguous radar "range" of about six miles because radio waves travelat about 10.7 microseconds (two-way) per mile and the time betweenH-sync pulses is 63.555 microseconds.

UP-LINK SUBSYSTEM.

A standard U.S. color TV frame consists of 525 horizontal "lines" and29.97 frames are transmitted per second to yield 15,734 horizontal lineswith H-sync pulses per second. Thus subscriber transponder up-linkreply-echos could be triggered by the TV horizontal sync pulses at arate up to 15,734 pulses per second. However, this data rate is muchfaster than typical transponders require because they are usuallydesigned for a performance comparable to telephone modems (300 to 1200bits per second). In one embodiment of the invention the transponderstransmit a RF reply pulse to send a logic "1" or no pulse (no emission)for logic "0" when interrogated by a TV H-sync pulse. Consequently, iftransponders are designed to transmit at 300 baud they will respond(i.e. provide an echo) on every 52nd TV H-sync pulse (15,734/300 ≐52).

Several transponders could therefore be scheduled to initiatetransmission on different TV "lines", that is, different H-sync pulses,and on every 52nd horizontal line thereafter (modulo 52). For example,one transponder could use the H-sync pulse of TV horizontal lines 1, 53,105, . . . 469. Another transponder at the same location could beprogrammed to transmit on horizontal lines 2, 54, 106, 158, . . . 470,and so on. Thus up to 52 different subscribers living at the same rangecould effectively transmit on the same subchannel from one range "cell"(i.e. one range gate) location, but operating on 52 different TV H-synclines of the 525 horizontal lines available on each TV frame. Thispermits each of them to send 10 bits on each TV frame, which results inabout 300 baud transmission rate each.

This multiplex method is defined herein as a "coarse" time divisionmultiplex process to distinguish it from the "fine" time division(actually space division) multiplex process that occurs becausedifferent subscribers live at different distances from the TVtransmitter, thus at different transit time (range gate) intervals.

For example, a system using six range gates, each five microsecondswide, would provide an unambiguous T-NET service area radius of aboutsix miles. Thirty-two different radio frequency subchannels could becreated in one vacant 6 MHz wide TV channel. Consequently 9984 differentsubscribers (52 H-sync lines×6 range cells×32 subchannels) couldsimulataneously operate in one angular sector without signal "clashes"or range ambiquity. A similar number could operate in a 12, 18, or 24mile service radius using a software routine to eliminate the "radarrange ambiquity" which arises when more than one H-sync pulse is intransit at one time between the TV transmitter and the subscribers.

At the central receiving site each basic timing process commences witheach TV frame. This occurs upon receipt of a vertical synchronizing(V-sync) pulse from the television station and the 525 H-sync pulsesthat follow it (in the U.S. standard NTSC format). In a preferredembodiment the H-sync pulses each trigger the start of a "range addressgenerator" at each central receiver which generates a series of delayedreceiver range gates, each having a width of 5 microseconds. This widthis adjusted to match the width of the pulse signal transmitted from eachsubscriber. Five microseconds is also the approximate width of thestandard TV H-sync pulses. The central receiver, which has previouslystored in its memory the range to each subscriber transponder, opens upa range gate at the expected time of arrival of each transponder digitalbit pulse to thereby determine if the transponder has sent a logic "1"(i.e. a transmitted pulse) or a logic "0" (no transmitted pulse). AT-NET system serving a 24 mile radius would expect a 257 microsecondmaximum duration between receipt of a TV H-sync pulse and receipt of adelayed transponder reply pulse if the TV transmitter and the centralreceiver ar co-located.

A computer at the central receiver collects all of the time interleaved"0" and "1" responses from its many subscribers, sorts and groups theminto separate packets and appends the appropriate subscriber address.One preferred packet structure which can be employed in the invention isthe so-called X.25 public packet switching protocol which is expected tobe universally accepted. These packets are then forwarded throughconventional communication trunk lines to a centrally located networkcontrol center (NCC) where the packets are further routed byconventional means to various Information Providers such as data banks,electronic mail services, financial institutions, and the like. Theirreplies are similarly routed back to each subscriber as described below.

DOWN-LINK SUBSYSTEM

Present Art. In the instant invention, digital communications totransponders could be superimposed on a television signal using eitherconventional techniques known today (e.g. Teletext) or the enhanced newmethods disclosed in this specification. In the present art, teletextdigital signals are transmitted on up to 8 of the 21 TV horizontal linesthat lie within the vertical blanking interval (VBI) of conventionaltelevision signal formats. One such method is called the North AmericanBasic Teletext Specification (NABTS) and this standard permitsapproximately 288 bits of information to be packed in each of eight ofthe twenty-one horizontal lines that lie within the TV vertical blankinginterval. Since the VBI repeats at the rate of 60 times a second, thisresults in an average down-link traffic capacity of approximately138,000 bits per second (288 bits×8 lines×60 Hz). We refer to thesemethods as co-channel techniques because they lie within, and share theHost TV's channel.

New and improved means are taught in this specification for sendinginformation down-link to subscribers at greater speeds and morereliably. These are subdivided into two classes: (1) those that operateon the adjacent upper or lower channel (or both), and (2) co-channeltechniques that share the same channel as the TV Host station. Theinventor's improvements are summarized in the following paragraphs.

Adjacent Channel Down-Link. A preferred embodiment of the presentinvention packs 4 bits of information on each of up to 32 time-gatedsubcarriers. These subcarriers are tuned to a channel adjacent to theHost TV station channel and are gated to exist only within itshorizontal blanking interval (HBI). Since the HBI repeats at the rate of15,734 times per second, this provides a potential capacity ofapproximately 2 million bits per second (15,734 Hz×4 bits×32subchannels); a substantial improvement over existing Teletext. Whilethis method requires use of a vacant adjacent channel above or below (orboth) the host TV station, some of the same subchannels used for theT-NET up-link can be used for the down-link as well, even on the samesubchannel at the same time.

Co-channel Down-Link. The second improved method to increase the digitaltraffic capacity of down-link data streams superimposed on the TVtransmission is taught here. It uses the same channel as the TV signal(co-channel) and involves the sequential adding and subtracting ofidentical digital data streams to the existing video picture informationat corresponding TV picture elements of sequential TV frames. Theprocess is as follows: the existing TV video information of eachhorizontal line of a first frame is stored and compared to the video onthe corresponding lines of the following frame to locate non-moving("frozen") portions of each scene. The desired digital data is firstadded then subtracted on corresponding lines of the first and secondframe, preferably at only the frozen scene portions. For example, eachdigital bit added to each line could be a pulse about 185 nanoseconds induration as in existing Teletext to yeild 288 bits per line. On thefollowing frame the same digital data is inverted and, in effectsubtracted from the frozen video of the previous correspondinghorizontal lines and again transmitted. The result is that at any TVpicture spot (pixtel) of the television viewer's screen the digitalinformation which is first added then subsequently subtracted, cancelsand becomes invisible. Each of the 525 lines per frame could carry datain this manner. Note that each frame consists of 525 lines in twointerleaved "fields" of 261.5 lines each in the U.S. Standard.

The invisibility of data is primarily due to the known psychologicalcanceling process of human vision, but also because the phosphors of thetelevision screen have a slight averaging effect that smoothes out TVscenes. This cancelling effect can be optimally adjusted for dataadded/subtracted from fixed televised scenes as well as to minimize"beat" effects caused by its presence with the color TV chromasubcarrier. However, for televised scenes that include motion there is aslight difference in the video level from one frame to the next in themotion part of each scene and therefore superimposed digital datafollowed by inverted data, may not completely cancel. Fortunately, ifthe data is transmitted at the high rate proposed here, one cancapitalize on the fact that the frequency response of the human eye tothis "high fidelity noise" is masked by the motion in those portions ofthe moving scene. In other words, the human eyes' resolutiondeteriorates and does not see high frequency extraneous components of ascene which is in motion and one could send data with motion scenes withsome sacrifice of picture quality. Alternatively, data transmissioncould be inhibited in scene segments which contain motion as suggestedabove.

The technique of adding and subtracting digital data just described canbe implemented using known digital TV scene store and forwardtechniques. This is rather simple on televised black and white programs.The description of how the new process works on color televisiontransmissions is somewhat more complex, though essentially similar aswill now be described.

Color television basically transmits three different signals related tothe primary colors (red, green, blue) and these are generally refered toas the in-phase (I), quadrature (Q) and luminance (M) components. Forreasons related to characteristics of human vision it turns out that thefrequency bandwidth requirements of the luminance component "M" issubstantially greater than the other two. The "I" and "Q" components arein fact superimposed on a chroma subcarrier channel having a usefulinformation bandwidth of only one-third to one-half that employed forthe luminance component. Consequently, the instant invention providesfor the modulation of digital data on the luminance component only andin such a manner that the digital data manifest frequency spectra wellabove the spectra of "I" and "Q" components, thus invisible to them, andin such a manner that superimposed digital data followed by inverteddata superimposed on corresponding locations of a following framevisually cancel substantially as described before. Thus regular TV videoand piggyback data may be transmitted simultaneously on the same TVchannel.

This process could be accomplished at a data rate representing theproper harmonic ratio of the horizontal sync rate to optimize visualcancellation in much the same manner used to select the proper TV chromaoscillator frequency in present day color TV receivers.

This improved method of piggyback down-link co-channel transmission isparticularly attractive for such applications as video conferencingwhere a speaker in a central location may wish to address a large numberof remotely located offices and in which he uses a series of charts andgraphs; thus a large part of the TV scene comprises low data contentfixed video consistent with the capability of this invention which isslower than regular TV. Most of the motion is primarily in the speaker'slips. This point-to-multipoint video conferencing mode superimposed onregular TV is yet another attractive application of the T-NET system andhas the additional benefit of having a return path so that the listenerscan "talk back".

Angular Sectors. One preferred embodiment of the instant invention wouldemploy directional antennas at the central receivers, for example, eachhaving a gain of approximately 20 dB and beamwidth of about 18 degreesat UHF. Twenty such antennas would provide a full 360 degreeomni-directional coverage if all were located in one central location.Central receivers could be located near the host television transmitteror they could be dispersed throughout a city area depending upon thelocal topography and coverage desired. In one preferred embodimentpreviously described, each subscriber transponder transmits for aboutfive microseconds when it is interrogated by every 52nd H-sync pulse.The radio frequency (RF) bandwidth required to carry such a signal is onthe order of 187 KHz. Thus 32 different transponder "subchannels" couldbe assigned within one typical 6 MHz television channel. For example,sixteen even numbered transponder subchannels could be assigned to onedirectional receiving antenna sector while the adjacent antenna sectorscould use the 16 odd numbered subchannels. Such a plan would permit there-use of the even and odd numbered transponder subchannels many timeswithin a city to significantly increase the overall system digitaltraffic capacity.

Vehicle Location. Because this invention operates in a manner analogousto a radar system wherein the TV horizontal sync pulses are equivalentto the radar's outgoing transmission pulse, and where the transponderpulses triggered by it comprise a reply echo, it is clear that distanceto each subscriber can be accurately determined. This is used toadvantage in two ways: for fixed subscribers the central receiver canaccurately predict the time at which each subscriber transmission pulsewill be received, consequently it can optimumly schedule subscriberresponses in a space, time and frequency division manner to optimize thesystem traffic capacity. Alternatively, if the range to each subscriberis unknown, as for example in portable or vehicle mounted devices, thenspecific frequency subchannels can be dedicated to those mobiletransponder applications so that one can measure the position of eachvehicle and automatically keep track of its location using targetacquisition and tracking techniques well known in radar. This is donesimultaneously with data transmission with the vehicle.

In the illustrative system previously described, the angular bearingmeasurement to the unknown vehicle position would be rather crudebecause of the relatively wide beamwidth (i.e. 18 degrees). On the otherhand, the range to transponders can be precisely measured to the orderof a hundred feet or so. Consequently, a T-NET system can be optimizedto provide much better vehicle location accuracy by using two separatedcentral receivers properly programmed so each measures range to eachmobile transponder and thereby more accurately determines vehiclelocation (about 300 feet accuracy is anticipated).

CATV Application. Yet another application of the invention lies in thearea of cable television (CATV). The isolation of signals to and fromsubscribers in present day CATV systems has been found to be a problem,partly because of the fact that when many subscribers transmitters areconnected to the television cable they each contribute undesirablenoise. Since this noise is additive in present day continuous wave (CW)techniques, the cumulative noise of all two-way CATV subscribers pose aserious problem. The instant invention solves this problem inessentially the same manner as described above for over-the-airapplications. The horizontal blanking interval of a cable TV program ison the order of one mile as in the examples discussed before. In theinstant invention the transponders' up and down-link emissions existonly during the HBI, hence are invisible to subscribers living within amile of each other. One embodiment of a cable TV application of thisinvention would install T-NET "master" (multiplexed) repeaters withinCATV amplifier boxes which typically are at intervals on the order ofone mile along the TV cable, and regular transponders at eachsubscriber's home. T-NET signals are collected at the master repeaterand relayed "over-the-air" to a T-NET central receiver and processed inessentially the same manner previously discussed.

The various techniques just described comprise the essential buildingblocks from which various system architectures may be devised topractice this invention. For example, it is obvious each transpondercould reply to all H-sync pulses to provide a 15,734 baud rate andthereby "burst" its up-link message much faster; combining this with adifferent time-sharing arrangement between transponders provides yetanother mode of operation. It is also obvious that if the Hosttelevision station went "off the air" at night, or for whatever reason,a separate independent transmitter (perhaps owned by the T-NET operator)could be put on the air to broadcast the necessary H-sync and V-synccoordinating signals. In such instances concern for protecting viewerswould be irrelevant. Thus the various applications described herein andothers will become evident to the skilled communications system designerupon careful study of the operating details of these building blocks ashereinafter described.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will become apparentfrom the following specifications taken in connection with theaccompanying drawings, wherein like reference characters identify partsof like functions throughout the different views thereof.

FIG. 1 is a block diagram of the overall T-NET system configuration.

FIG. 2 is a pictorial of the invention employing directional antennas tosegment the service area into pie shaped sectors covering a city areaand illustrating possible locations for a receiver substation.

FIG. 3 is block diagram and pictorial illustrating the manner in whichTV horizontal sync pulses trigger subscriber transponder replies and apictorial illustrating the appearance of the TV sync pulses followed byrange delayed subscriber replies, as in a radar "A" scope.

FIG. 4 is a plan view of one central receiver directional antennacoverage sector segmented into range cells.

FIG. 5 illustrates the manner in which the television horizontalblanking interval (HBI) is superimposed on, and thus masks thesubscriber transponder pulse transmissions.

FIG. 6 is a graph illustrating the rapid drop-off (attenuation) in thestrength of the subscriber transponder signal pulse with propagationdistance.

FIG. 7 is a top view of one typical communication path between TVstation and subscriber transponder, illustrating the area blacked outduring the HBI.

FIG. 8 is another graph illustrating the typical signal levels found inthe HBI of a standard television waveform.

FIGS. 9A-9C illustrates the gated subcarrier down-link adjacent channelembodiment.

FIGS. 10A-10C illustrates two methods of modulation which can beemployed in gated subcarrier down-links.

FIG. 11 is an illustration of the invention as applied to simultaneousup-link and down-link operation on the same subchannel.

FIGS. 12A-12B is a block diagram of one embodiment of a typicalsubscriber transponder.

FIG. 13 is a block diagram illustrating one way in which an antennaduplexer may be constructed to permit sharing of an existing TV antennabetween the transponder and the existing television receiver.

FIG. 14 is a block diagram of a radio central office.

FIG. 15 is a block diagram of one embodiment of the central receiver.

FIG. 16 is a block diagram of one embodiment of the digital interfacecircuits section of a central receiver.

FIG. 17 is a block diagram of the invention as applied to providetwo-way cable television.

FIG. 18 is a block diagram of the invention as applied to automaticvehicle location, including a digitized "slow voice" up-link.

FIG. 19 illustrates the co-channel down-link digital video transmissiontechnique.

FIG. 20 illustrates the application to cellular radio.

DETAILED DESCRIPTION OF THE INVENTION

Reference now should be made to the drawings in which the same referencenumbers are used throughout the various figures to designate the same orsimilar components.

FIG. 1 illustrates the major components of an entire system of theinvention for three applications serving a large number of: mobiletransponders, radio modems used in conjunction with personal computers(PC), and two-way interactive television viewers having remote hand-heldcontrol means. The system of FIG. 1 is intended to providecommunications facilities for a plurality of host computers 4 whoprovide information to subscribers, or so that one or more Hosts, actingas switch centers, may establish what are sometimes called virtualcircuits that enable subscribers to communicate with each other. Theprincipal device used by subcribers to communicate on the system of thisinvention comprise receiver-transmitter devices usually referred toherein as "transponders" but sometimes called "radio modem" or "RFmodems" when used with personal computers.

Referring to FIG. 1, the network control center 2 employs conventionalcomputer hardware, software and trunk lines 26 to receive, temporarilystore, route, and forward digital messages between the host computers 4,the broadcast station interface unit 8, and radio central offices 6. Forexample, a subcriber at a fixed location 15 sitting at personal computer20 may communicate digital information packets through radio modem 14via radio signals transmitted through antenna 12 to a centrally locatedantenna 28 and radio central office 6 which detects and reformats thesemessage into standard packets and forwards them to network controlcenter 2. The network control center reads the destination addressportion of these packets and forwards them to the appropriate hostcomputer 4, which is one of a plurality of hosts. If a reply isrequired, the host computer 4 generates the reply message and sends itto the network control center 2 where it is reformatted and placed intoa transmission queue where, at the appropriate time, broadcast stationinterface unit 8 transmits it over TV transmitter station 10 where themessage is radiated over the air and detected by antenna 12, demodulatedby radio modem 14 and sent to personal computer 20 to complete themessage loop.

The transponder device is functionally the same whether it isincorporated within a plurality of mobile subscriber packages 25, radiomodems 14, or integrated in interactive televisions 16. Device 25 couldbe a portable computer terminal or simply a "Two-Way Pager" which hasthe added benefit of being able to acknowledge "beeps", or even send andreceive alpha-numeric messages. If the subscriber transponder isintegrated within television 16 then it may be conveniently operatedthrough a remote hand-held device 18, which could communicate withtelevision 16 using conventional wireless techniques, such as infraredsignalling, thus providing "Interactive TV". The antennas 28 employedwith radio central offices 6 may be directional in design so that eachreceives only from a specified direction and thereby partitions theservice area into pie shaped sectors. The communication paths 26connecting the various components of the common central equipmentcomprise conventional communication trunk lines such as microwave links,dedicated phone lines, or other suitable means.

FIG. 2 is a pictorial illustration of the invention using a plurality ofdirectional antennas to cover a city area from a radio central office 6and a receiver substation 7, which substation is essentially the same as6 but displaced from it so as to extend coverage into areas that may notbe accessable to radio central office 6 because of mountains or otherobstructions. FIG. 2 also illustrates the definition of a radio cellwhich, for purposes of this specification, is considered to comprise ageographic area defined by the beamwidth of each antenna 28 and a rangegate interval such as the distance between R2 and R3. It will be pointedout in subsequent discussion that the distance between R2 and R3 isproportional to the propagation distance covered during the subscribertransponders pulse-width which is on the order of five microseconds in apreferred embodiment. Consequently the distance between R2 and R3 is onthe order of a mile.

FIG. 2 also illustrates how network control center 2 may be providedwith intercity communication means through use of satellite commnicationlink 30. Messages may be communicated between cities by these and otherwell known methods.

FIG. 3 is intended to facilitate the explanation of essential featuresof the invention. For purposes of illustration, operation of theinvention is considered analogous to the operation of a radar system. Aradar system typically transmits brief radio pulses which impinge on"targets" in its transmission path that reflect back pulse energy(called the echo in radar) that is detected at a receiving point afteran elapsed time t. The elapsed time t is proportional to the pulsepropagation distance to and from the reflecting targets and consequentlydistance to targets (subscribers) may be determined by measuring t. Whendirectional antennas are employed to either send or receive signals (orboth) then the direction to the target may also be determined.

Referring to FIG. 3, TV transmitter 10 radiates a conventionaltelevision signal including horizontal sync pulses 31 and digital datawhich are detected by the transponder antenna 12 and sent throughantenna duplexer 32 to the receiver 34. Receiver 34 locks on to the TVsignal and extracts from it the horizontal and vertical synchronizingpulses which are subsequently employed to detect the T-NET digitaldown-link signals that are synchronized to and accompany the TV signaland also to coordinate the radio modem's reply pulse transmission timeslots so reply pulses exist only in the HBI. The digital information andsynchronizing pulses are connected to microprocessor 36 where theaddress portion of the message packet is examined to determine if it isa signal intended for that specific subscriber. If it is, it isforwarded to the companion personal computer 20. The link betweenmicroprocessor 36 and computer 20 could employ the well known RS-232standard. Computer 20 digests that information and if a reply isnecessary it will generate it and transmit it back to microprocessor 36where it is temporarily buffer-stored and prepared for transmission atappropriate time slots using transmitter 38. Transmitter 38 generates anRF pulse in synchronization with the horizontal sync signal time slotreceived from microprocessor 36 and transmits that pulse throughduplexer 32 and antenna 12 back to the central receiver antenna 28.Antenna 28 may be one of a multiplicity of directional antennas toprovide the desired city coverage. Antenna 28 is connected to the radiocentral office 6 where the up-link information is detected, reformatted,and sent to the network control center 2 illustrated in FIG. 1. Anotherantenna 27 at the radio central office detects sync signals fromtelevision station 10 and connects them to the radio central officewhere they are employed to initiate the desired timing processes basedon the TV signal's horizontal and vertical synchronizing pulses.

Referring again to FIG. 3, the "folded A scope" receiver monitor shownin the center of the illustration is intended to facilitate thedescription of the T-NET system operation in analogy to a radar system."A" scopes are commonly used in radar to display range to varioustargets. In these examples the trace is "folded" into many lines. The"A" scope monitor shows a series of horizontal line sweeps (like a TVraster scan) each of which starts when it is triggered by the horizontalsync pulse transmitted by TV station 10. This is called the start pulsein that illustration and a short time later an echo pulse from asubscriber appears; the time duration t between the start pulse and theecho is indicative of the range to that subscriber.

The first line of the folded "A" scope is called H1, the second H2, andso on through H52 for a modulo 52 system. Since each line is triggeredby H-sync pulses from TV station 10, each line has a duration of 63.555microseconds in the U.S. TV standard. The length of each line thuscorresponds to a distance of about 6 miles. This folded "A" scopemonitor may also be viewed as equivalent to a TV screen rastor scanwhich sweeps out 52 lines then repeats itself.

In radar jargon each horizontal line is called a "A" scan but in thisillustration, having many horizontal sweeps, we refer to it as a folded"A" scope. The folded A scope monitor shown in FIG. 3 could be employedwithin a radio central office 6 for the purpose of monitoring the radiosignal activity, or perhaps for technical evaluation or trouble shootingand to show at a glance the strength of, and range to, varioussubscribers. In actual practice the detection, storing, and routing ofsignals is all done automatically by computers and such a display wouldnot be required for those functions.

FIG. 4 is a top view of one angular sector of a radio central officeservice area. Antenna 28 provides reception of signals in an angularsector approximately 18 degrees in width and that sector is furtherpartitioned into range cells numbered from 1 through 6 in the firstrange interval, and similarly in the second and third range interval.Each of these range cells is one mile in length and this corresponds toa time duration of 5 microseconds, which duration is also the width ofeach transponder reply pulse.

It was pointed out earlier in this specification that the unambiguousrange of the T-NET system is proportional to the time duration betweenTV horizontal sync pulses and this turns out to be 63.555 microseconds,about 6 miles. This is called the first range interval. The second rangeinterval, also numbered 1 through 6 extends from 6 to 12 miles and thethird range interval extends from 12 to 18 miles. Obviously, the numberof range intervals required depends on the size of the city.

Since each TV signal horizontal sync pulse is numbered from 1 to 525starting from the first vertical sync pulse which defines a TV frame, itis clear that the central receiver 6 as well as each subscribertransponder, is each capable of unambiguously counting and keeping trackof all 525 horizontal sync pulse numbers. Consequently it is clear thata software algorithm can be devised to remove any ambiguity that mightexist as to whether a subscriber lives in range interval 1, 2, 3, etc.,and which specific H-sync pulse they have been assigned to reply on.

It was also pointed out earlier that about 10,000 subscribers couldoperate simultaneously at 300 baud each within one six-mile rangeinterval. That maximum number of subscribers would remain the same eventhough more range intervals might be employed to yield a 12, 18, or 24miles service area. The number of subscribers could be increased,however, by using more angular sectors or more subchannels. For example,if 18 degree beamwidth antennas are used to cover a 360 degree area,then a total of 20 antennas would result and this would service close to200,000 subscribers simultaneously. However, in a practical world thesubscribers are not uniformly distributed throughout a service areabecause of terrain and service boundaries, therefore less than theoptimum number of subscribers could be serviced simultaneously in apractical system. Obviously a much greater number could be served on atime-shared basis because each subscriber typically use the system onlymomentarily for a few minutes per day.

FIG. 5 illustrates in a series of microsecond time-steps the manner inwhich subscriber transponder pulses are masked (i.e. rendered invisibleto TV viewers) by the horizontal blanking interval of the Hosttelevision signal. The television station is assumed to be at the leftside of FIG. 5 and its signal is assumed to propagate from left toright. When the Host television signal H-sync pulse impinges on thesubscriber's television antenna, it triggers the generation of atransponder reply pulse (echo); the leading edge of this reply pulse isillustrated in FIG. 5a as a straight vertical line beginning at theleading edge of the 11 microsecond square pulse labeled "HBI". All thisoccurs at initial time t.

FIG. 5b illustrates the almost fully developed reply pulse at a timet+4.5 microseconds. The subscriber's reply pulse is shown in diagonallyshaded lines (so long as it is under the HBI pulse) and it is seenpropagating both to the left and to the right, upstream and downstream,respectively. Since radio waves travel at approximately a thousand feetper microsecond, the subscriber's reply pulse would have propagatedapproximately 0.9 miles within 4.5 microseconds and, if thisillustration was seen from the top view, one would see that thesubscriber's reply pulse would represent a circle 1.8 miles in diametercentered on the subscriber's antenna. The square pulse labeled HBI abovethe shaded subscriber's pulse is the Host TV horizontal blankinginterval and it is seen to propagate to the right at the same speed asthe subscriber's pulse and all of the energy of the subscriber's pulsetraveling that direction is seen to exist within the horizontal blankinginterval. It will always do this in the downstream direction.

FIG. 5c illustrates the time waveforms as they exist at t+8microseconds. Since the subscriber's pulse is only 5 microseconds wideit is seen that the subscriber's transponder has ceased transmitting andthe reply pulse trailing edge has left the subscriber's antenna and isnow propagating in all directions. If this illustration were viewed fromthe top, the reply pulse would appear as a doughnut with an outsidediameter of about 3.2 miles and an inner diameter (hole) slightly over 1mile in diameter, all centered on the subscriber's antenna. Note theimportant point that waves propagating to the right (downstream) stillexist underneath the horizontal blanking interval of the Host TV signalbut the waves traveling to the left (upstream toward the TV station) areno longer masked by the HBI; in other words, TV viewers who liveupstream more than about 1.2 miles from the subscriber are notsimultaneously blanked-out and they could, if the subscriber's signalwas strong enough, see the subscriber's response pulses. We will pointout shortly that the subscriber's pulse is quite weak by the time itreaches that distance.

FIG. 5d shows the waveforms which exist at t+18 microseconds at whichpoint the subscriber's pulse waveform which propagates downstream, shownin cross-hatched shading, still remains under the HBI and is thusmasked, but the pulse propagating upstream (to the left) shown withoutcross hatching, is about 3.4 miles upstream and, since it is out of theHBI, could be seen by TV viewers, if it were strong enough.

FIG. 6 shows graphically a plot of the signal strength of the subscribertransmission pulses as a function of distance from the subscriber. It iswell known in radio wave propagation theory that in free space theelectric field intensity of a propagating radio wave falls linearly inproportion to the propagation distance. The power in that wave falls asthe square of the distance and is plotted in FIG. 6; the radio wavepower drops very rapidly in the first few hundred feet after leaving theradiating antenna and more slowly thereafter. By the time the radiowaves reach a distance of approximately 500 feet they have fallen inmagnitude by about 70 dB. The point of FIG. 6 is to show that thestrength of subscriber's transmitting pulses drop so rapidly in thefirst few hundred feet to a level which becomes insignificant incomparison to the strength of TV radio waves and thus would notinterfere with television signals.

Consequently television viewers watching programs sent by the Hosttelevision station must be protected only if they live within a fewhundred feet of the subscriber's transponder antenna because this iswhere the transponder signal is strong and potentially capable ofinterfering with the television program. Fortunately, as shown in FIG. 5illustrations, television viewers living within a few hundred feet ofthe transponder are simultaneously blanked out by the horizontalblanking interval of the Host television signal and consequently eventhough the subscriber's transponder signal is relatively strong andpotentially capable of interfering with adjacent channel televisionviewers, all those viewers' television receivers are blanked out andcannot see any video program at that instant.

By the time the subscriber's signal propagates to a distance outside thehorizontal blanking interval it is about 90 dB weaker (i.e. about 1billion times weaker) and will not interfere with the Host television'ssignal. Furthermore, since the subscriber's transponder operates on achannel adjacent to the Host television signal, it is suppressed furtherby radio frequency filters which are tuned to the Host television signalrather than the transponders signal and this suppression typicallyamounts to 35 dB or more. In other words, a television receiversuppresses adjacent channel signals by about 35 dB or more. The combinedeffect of the signal propagation attenuation shown in FIG. 6 and theattenuation due to the television receivers tuned circuits total over115 dB. Therefore, for all practical purposes the subscriber'stransponder signal cannot interfere with television viewers tuned to theHost television signal.

In a practical environment (not "free space" as plotted in FIG. 6) thesubscriber's transponder pulse attenuates even more rapidly and so thesignal attenuation is even greater than 115 dB. TV viewers tuned tochannels further away than the first adjacent channel suppresstransponder pulses 50 dB or more because of their tuned circuits andthey too are uneffected as has been demonstrated in many field trials.

FIG. 7 is a top view illustrating the HBI masking geometry and itcorresponds to the side view shown in FIG. 5. The horizontal blankingpulse 31 is shown propagating outward as a circular wave centered on TVstation 10. The cross-hatched area shown around subscriber 15 is thearea which is masked by the Host TV horizontal blanking interval (HBI)and all television viewers living within that cross-hatched area wouldnot be able to see the subscriber's transponder pulses because thescreen of their television receiver is blanked out by the HBI at thatmoment. On the other hand, television viewers living to the left of thesubscriber in the area which is not cross-hatched would not be protectedby the HBI masking but would, on the other hand, be protected by thevery low subscriber signal strength which has already been discussed inrelation to FIG. 6.

We have thus shown that transmissions from subscriber transponders wouldnot cause interference to television viewers looking at the Hosttelevision signal because they are either blanked out by its horizontalblanking interval if they live close to the subscriber, or they are tooweak to interfere with the TV signal if they live outside the horizontalblanking interval since they would then be at least a mile away.

There is another potential concern however and that is the question ofwhether the subscriber's transponder signal may somehow interfere withcertain television receiver functions which must be accomplished duringthe horizontal blanking interval. We shall now address that point.

FIG. 8 shows a standard television waveform defined by the NTSC(National Television Standard Committee) during the horizontal blankinginterval. Time is assumed to start at the left and increases to theright. Thus the first feature one sees after the start of the HEI is thefront porch just preceding the horizontal sync pulse. That front porchis used to define a reference for the so-called black level; signalsweaker than that level lie within the visible range of the TV screen andsignals stronger than that level are black and cannot be seen. Thus thehorizontal sync pulse, which is stronger than that reference level,cannot be seen. Since the subscriber's transponder commencestransmission with the beginning of the horizontal sync pulse it is clearthat it cannot interfere with the use of that reference black levelbecause it exists prior to the beginning of the horizontal sync pulse.The TV horizontal sync pulse itself triggers the subscriber'stransponder as well as all of the circuits needed by televisionreceivers at the subscriber's home or in its neighborhood. Thus asubscriber transponder signal occuring at this point would only appearlike a regular horizontal sync pulse and it does not intefere with theproper horizontal sync of any television receivers in its vicinity.

Following the horizontal sync pulse (FIG. 8) a "chroma-burst" waveformis transmitted on the back porch in the case of color TV signals. Thatchroma-burst represents approximately 8 cycles of a chroma subcarrieroscillator operating at a frequency of about 3.57 MHz and its purpose isto synchronize crystal-controlled oscillator within each televisionreceiver which is used to demodulate the color signals. Interferencewith that process could cause degradation in the color balance of colorTV programs. Television receivers universally employ acrystal-controlled chroma oscillator tightly locked to that chroma-burstand this acts as a very sharply tuned filter. The filter is in fact sonarrow in bandwidth that the broadband energy density of thesubscriber's transponder pulse used in this invention has minimal effectupon it. In other words, the spectral power density (watts per hertz)represented by the subscriber's transponder pulses as used in thisinvention is of such low value that the very small amount of energywhich does exist within the very narrow TV chroma bandpass filter oftelevision receivers is insufficient to interfere with it. Numerousexperiments conducted by the inventor have shown that the transmissionscontemplated in this specification have no effect on the color qualityof television programs, even if transponders and TV receivers share thesame antenna.

We have thus shown why transmissions by subscriber transponders designedin the manner set forth herein will have no deleterious effect ontelevision viewers in the neighborhood of the subscriber or elsewhere,even if they share the same antenna as the subscriber's own televisionreceiver.

One may question, however, how weak the signal must be in order to notinterfere with television signals. Those issues and relatedspecifications are determined by the Federal Communications Commissionin the United States and by similar agencies in other countries. At thepresent time, the FCC has stipulated that an adjacent channel signalmust be equal to, or weaker than, a television signal so as not tointerfere with it. Stated another way, the only protection afforded tothat television signal is the protection provided by the TV receivertuned filters which, as stated before, represents about a 35 dB or moreadjacent channel suppression. On the other hand, if a potentiallyinterfering signal lies within the same channel as the televisionsignal, then it must be weaker than the television signal by at least 50dB under existing FCC rules and only 40 dB under proposed new rules.Using existing FCC rules as a criteria, the inventor has found that asubscriber transponder may use a pulse power of approximately 2 wattspeak and average power of a few milliwatts to meet the FCC criteria andthis is also sufficient to provide a useable signal to a T-NET radiocentral office at distance exceeding 20 miles. Battery operatedtransponders appear practical because of the low average power of thesesubscriber transponders.

Thus far we have described the operation of the up-link from subscriberto radio central office. We have pointed out why subscribertransmissions do not interfere with television viewers. We have alsopointed out how the horizontal and vertical synchronizing pulses of theHost television signal coordinate the subscriber transpondertransmissions and permit many subscribers to be multiplexed on differenthorizontal lines of the TV frame. We shall now describe the theory andspecific advantages of the new and improved down-link from the Host TVstation to subscribers.

FIG. 9a shows the spectrum of the Host television signal and the 32 newdown-link subcarriers of this invention. In this illustration, they areshown to exist in the Host TV station's lower adjacent channel. It iswell known that television signals employ what is referred to as uppersingle sideband (SSB) modulation with a small vestigial lower sideband.The Host television signal carrier frequency Fc is shown at the leftside of the TV 6 MHz signal channel and most of the video energy isshown in the upper sideband. That signal energy comprises the videopicture information, a frequency modulated audio carrier at 4.5 MHzabove Fc and a color subcarrier at 3.57 MHz. There are also additionalsubsidiary carriers (SCA's) which could exist within the TV channel forthe purpose of providing stereophonic sound transmission and secondaudio programs but they are not shown in FIG. 9A. The lower edge of theTV channel is 1.25 MHz below the video carrier Fc. Frequencies lowerthan 1.25 below Fc are considered to be in the next TV channel, which isreferred to as the lower adjacent channel. As noted before, this hasalways been vacant. It is in this lower adjacent channel where the 32subcarriers of the instant invention are positioned (they could also useupper adjacent channel). The bandwidth of each of these 32 subcarriersis approximately 187.5 KHz and each of them is wide enough to carryindependent up-link pulse signals from subscriber transponders as wellas separate down-link signals to subscribers as will now be described.

FIG. 9b illustrates how 4 digital bits can be modulated within the timeinterval of a horizontal blanking interval which is approximately 11microseconds long (about 2.8 microseconds per bit). Each of thesubcarriers shown in FIG. 9b are gated so that they exist only duringthe HBI and consequently they will not interfere with the video portionof the Host television program. Within this HBI interval 4 bits ofinformation are modulated on each subcarrier. There are variousmodulation methods which may be employed to accomplish this. Onepreferred method is a phase modulation technique wherein the phase ofthe gated subcarrier is advanced 90 degrees and brought back to itsstarting phase within 1 bit interval when ever a logic "1" is to betransmitted. If more logic "1's" are to be sent in succesSiOn, the phasedirection is reversed after each "1" bit; that is the wave is advanced90 degrees and brought back to a starting phase within 1 bit intervaland then retarded 90 degrees and brought back to its starting phasewithin the 2nd bit interval. This is done in a sine wave fashion so asto restrain the signal spectrum as much as possible to keep most of itsenergy within its assigned subchannel bandwidth. If a logic "0" is to besent no phase advance or retardation will occur. This process is shownin FIG. 9b which illustrates a 4 bit sequence as 1011. Since 4 bits aretransmitted during each HBI for each subcarrier, and since the HBI'soccur at 15,734 Hz, this results in a down-link data rate of 62,936 bpsper subchannel. Each subchannel could carry information independently.

Quadrature Amplitude Modulation (QAM). An alternative modulation methodis disclosed in FIG. 9c which splits each subcarrier into quadraturecomponents and each of these components is independently modulated toprovide a more narrow transmitted spectrum and thereby minimizesinterference to adjacent subchannel signals. It will be clear to thoseskilled in the art that said quadrature method could employ eitherbinary (on-off) modulation of each quadrature term, or each term couldtake on multiple values (e.g. quadrature amplitude modulation: QAM) todefine multiple symbols for greater data rates per assigned subchannel.

Two different equipment arrangements for transmitting the down-linksubcarriers will now be described. FIG. 10a shows one method in which asubcarrier oscillator 40 followed by a frequency multiplier 42 generatesthe desired subcarrier radio frequency which is modulated by data in 44,amplified in 46 and radiated by antenna 48. This is one of thirty-twosubcarriers tuned to exist within the lower TV adjacent channel as shownin FIG. 9a. Those subcarriers are also gated to exist for only 11microseconds in the HBI. FIG. 10a shows an assembly of thirty-two suchsubcarriers generators. The output of all these subcarriers can besummed together in 50 and amplified by amplifier 46 and radiated throughantenna 48. Antenna 48 and indeed the entire assembly of FIG. 10a, couldbe independent and distinct from the Host televisiontransmitter/antenna. This particular method has the advantage thatantenna 48 could be a directional antenna. Several subcarrierassemblies, each identical to FIG. 10a, and their associated antennas 48could be provided to generate down-link transmissions into other angularsectors to thereby cover an entire city.

FIGS. 10b and 10c show two methods for transmitting down-link digitaldata. The RF subcarrier assembly of FIG. 10b, being already at theproper radio frequency, is "added" to the regular TV video carrier in 54and radiates through the TV transmitter antenna. This is a method topiggyback T-NET data signals on an existing Host TV transmitter withoutinterference because the T-NET subcarriers exist on an adjacent channelas previously explained. The Host TV transmitter may have to be retunedsomewhat to permit this, however. In FIG. 10c the subcarriers aregenerated at baseband frequencies and they in turn modulate the existingTV carrier in SSB modulators 11: the data carriers on the low sidebandand video on the upper sideband (with slight vestigial low sideband).

The alternative quadrature modulation method for impressing four bits ofdata on each subcarrier during each horizontal blanking interval fordown-link data transmission will now be described. The gated subcarrieroscillator 40 (FIG. 9c) is split into two quadrature components and eachof these components is modulated with two bits of data during each HBI.This is in contrast to the method described above wherein a singlesubcarrier component is modulated with four bits of data during the HBI.The method of using two quadrature subcarrier terms is attractive fromthe standpoint of minimizing the required radio spectrum bandwidth. Itis also very attractive because low-cost large integrated circuits(called IC's or "chips") now exist for "color" television receivers thatincorporate within them all of the circuits necessary to demodulate thechroma subcarrier and these can be adapted to demodulate the datasubcarrier instead, as well as to detect the horizontal and verticalsync pulses and necessary control signals (AFT and AGC).

The manner in which the down-link subcarriers are quadrature modulatedat the down-link transmitter end is relatively straight forwardReferring to FIG. 9c, the output of subcarrier oscillator 40 is splitinto two quadrature components by shifting one signal path 90 degrees inphase shifter 41. The in-phase and quadrature signal is then amplitudemodulated independently by 43 with 2 bits of data during each horizontalblanking interval. Of course subcarrier oscillator 40 itself only existduring the horizontal blanking interval (eleven microseconds) asexplained before. Thirty-two oscillators identical to FIG. 9c could beprovided for each antenna beam sector as previously explained.

It has been pointed out earlier in this specification that it ispossible to use the same subchannel to transmit up-link as well asdown-link, even at the same time. The use of separate subchannels foreither up-link or down-link transmission is fairly obvious. However, tounderstand the use of a single subchannel for both up-link and down-linktransmission at the same time requires some explanation. It has alreadybeen pointed out that the gated subcarriers used in the down-link existonly for approximately 11 microseconds coinciding with each Host TVhorizontal HBI. The time between sync pulses is 63.555 microseconds andconsequently the down-link subcarriers exist for only 17.3% of the totaltime (11/63.55=0.173). Hence the down-link subchannel is actually "off"and unused 82.7% of the time. As noted before, the HBI time intervalcontaining down-link subcarriers propagates away from the televisionstation with the speed of light and sweeps across the countryside to themaximum extent of the system's service area and beyond as shown in FIG.11.

Since these gated subcarriers are only on for 17.3% of the time thisleaves us with approximately 82.7% of the time free to listen for reply"echos". These echos are in fact digital up-link data as pointed out inthe prior discussion. In order to share subchannels for simultaneousup-link and down-link transmissions one must be careful to permit onlycertain fixed subscriber locations to operate in this manner so as notto cause the receipt of a reply pulse from prohibited locations (FIG.11) at the same instant a down-link subcarrier transmission is occuring.It would be difficult, if not impossible, to detect the weak "echos"from prohibited locations which arrive at the same time as one isgenerating a strong down-link transmission. These prohibited areas areshown in FIG. 11 as cross-hatched annular rings. The width of theserings, about one mile, represents approximately 17.3% of the totalservice range and occur every five miles. Subscribers located withinthese prohibited rings would not be able to us the same subchannel forsimultaneous transmission and reception, however, those subscribers inprohibited areas could use a different subchannel for transmission andreception as is customary in radio transmission. Alternatively, areceiver substation 7 (FIG. 2) could be positioned "down-stream" so asto effectively move its prohibited areas away from those of radiocentral office 6 and thereby provide continuous coverage.

Synergetic Modulation. A point of novelty in the instant inventionshould now be explained. It was pointed out in the discussion relatingto FIG. 9 that thirty-two subcarriers are positioned in the lowerchannel adjacent to the Host TV station signal (or alternatively on theupper adjacent channel). These subcarriers will in fact appear to theT-NET transponder receiver (FIG. 12b) as if they were lower sidebands ofTV carrier Fc, even though they may have been independently generated,and even though they may be transmitted from a different location thanthe TV transmitter. Another way of explaining this is to point out thatthe "beat" frequencies which result when both the subcarriers and the TVmain carrier Fc exist within the bandpass of the transponder receiver 34(FIG. 12b) and are processed by detector 88; the result comprisesenvelop-modulation, comparable to SSB modulation of carrier Fc by thesubcarriers. This envelope is demodulated by detector 88 as explainedshortly.

Since this process of effectively appending sidebands to an existingsignal (i.e. the TV carrier) to exploit its carrier energy and/or someof the modulation which it already carries e.g. H and V sync signals)appears to be a unique concept, it has consequently been labeled"synergetic modulation". Synergetic modulation is herein defined asfollows: The creation of psuedo radio sidebands on an existing radiosignal by means independent of the generator of that signal wherein saidmeans are located at the same or a remote location to thereby enhancethe reliability of the psuedo sideband transmissions and minimize mutualinterference.

Equipment Design Options. Specific T-NET equipment and systemconfigurations will now be described in detail. It will become evidentto skilled communication workers that many variations of the basic T-NETsystem design concept can be implemented for various applications andconsequently the following circuits and related illutrations representonly one preferred embodiment.

Transponder/Transmitter. FIG. 12 shows the RF subsections of a typicalradio modem (or transponder) which may be employed in the instantinvention. FIG. 12A, the transmitter section, is a relativelyconventional radio transmitter design employing a fixed referencecrystal oscillator 60 and a subchannel frequency synthesizer comprisingphase detector 62, low-pass filter 64, variable oscillator 66 andprogrammable divider 74; all of these being combined in a circuitcommonly called a phase-lock loop (PLL). The programmable divider 74 iscontrollable by the microprocessor 36 previously shown in FIG. 3. Thusthe subchannel frequency of the transponder is controllable by thatmicroprocessor and it in turn may be controlled by the remote networkcontrol center 2 (FIG. 1) so as to assign transponders to differentsubchannel frequencies dynamically at different times to optimizeoverall system traffic management.

The output of variable oscillator 66 is amplified and frequencymultiplied in 68 and pulse modulated in 70 by cosine squared modulator76. Modulator 76 is in fact a waveform generator that provides a pulsewaveform having a smooth attack and decay shape (e.g. cosine squared)and this is done to optimize the spectral content of the transmittedpulses so that most of their radio energy falls within the desiredsubchannel bandwidth. Alternatively, the output of 68 can be split intoquadrature terms, each term being modulated with one bit per HBI(equivalent to the down-link QAM method previously described). Thepulsed output of modulator 70 is further amplified in 72 to a level ofapproximately 2 watts peak and connected through duplexer 32 to antenna12 where it is radiated. Transmitted pulses are approximately 5microseconds wide and the duty cycle is very low; the resulting averagepower of the transmitter is about 1.5 milliwatts at 300 baud. This is avery low average power and is therefore attractive for battery-poweredoperation.

Transponder/Receiver. The transponder's receiver subsection is shown inFIG. 12b. It is intended to employ conventional integrated circuitsdesigned for mass produced "black & white" television receivers andconsequently uses relatively inexpensive and reliable piece parts. Analternative, using "color" TV circuits, is discussed later. Down-linksignals are intercepted by antenna 12 and are connected to TV tuner 80through duplexer 32. These signals are amplified in 82 and sent throughintermediate frequency (IF) bandpass filter/amplifier (BPF) assembly 84and connected to TV receiver integrated circuit chip 86. Receiver 86feeds back a control signal 87 to TV tuner 80 to provide automaticfrequency tuning (AFT). These are all conventional TV components; forexample, 84 could include ceramic IF filters used in TV receivers.Detector 88 demodulates the down-link signals and removes the RF carrierto provide the TV sync and subcarrier baseband signals to both low passfilter 90 and high pass filter 92 connected in parallel.

The intermediate frequency (IF) tuned circuits of the receiver in FIG.12B are tuned so as to encompas all thirty-two T-NET subcarriers as wellas the television carrier Fc. Since the television signal includes lowervestigial sidebands below fc, they are included within the bandwidth ofthe receiver and are demodulated. Consequently, the output signals fromdetector 88 include all of the thirty-two subcarriers as well as most ofthe Host TV horizontal and vertical synchronizing pulse energy becausethat synchronizing pulse energy exists in the lower frequency componentsof the TV signal and passes through low pass filter 90. It consequentlyappears at the output of filter 90 as the H and V sync shown in FIG.12B.

On the other hand, the data subcarriers exist between 1.25 and 7.25 MHzbelow the TV carrier Fc and they are filtered out by high pass filter 92and sent to mixer 94 where a phase-lock loop arrangement provides forthe selection and demodulation of only one of the thirty-twosubcarriers. That phase-lock loop operates as follows. Frequencysynthesizer 98, dynamically controlled by microprocessor 36 (FIG. 3),selects which of the thirty-two subcarriers will be demodulated.Frequency synthesizer 98 may be controlled by either companion devicessuch as a personal computer in the case where the receiver is inside anRF modem, or by the system network control center (NCC) in the samemanner as it can control the progammable divider 74 of the transmittersection. In any event, frequency synthesizer 98 controls voltagecontrolled oscillator 96 to set it at a specific frequency preciselyequal to the subcarrier frequency which is to be demodulated; thisprocess occurs in mixer 94, low pass filter 108, amplifier 110,frequency control varactor 102, and crystal oscillator 100. Theiroperation is identical to the operation of a common phase-lock loop(PLL) which is well known. The result is that VCO 96 is kept preciselyin tune with, and precisely in phase-lock with the average phase of thesubcarrier which is to be demodulated. Phase fluctuations in theselected subcarrier will be smoothed out by low pass filter 108.However, fast phase fluctuations, which will represent the desired phasemodulated digital data, are passed through low pass filter 104 andamplifier 106 and are sent to the microprocessor 36 (shown in FIG. 3).It will be recalled that the subcarriers are each gated to exist foronly 11 microseconds and coincide with the HBI of the television signal.Within this HBI interval four bits of information is phase-modulated inthe manner previously described in connection with FIG. 9. Thus theoutput of the receiver of FIG. 12B provides the H and V sync pulses ofthe Host TV signal as well as the down-link digital data in any one ofthe thirty-two subcarriers of the down-link subsystem.

The strength of the Host TV H-sync pulses coming out of filter 90 isindicative of the radio path attenuation between the Host TV and thesubscriber. It is therefore a measure of the power required in thereturn (up-link) path. Based on the principal of reciprocity, powerlevel control 77 (FIG. 12) provides a control signal to modulator 76which establishes the desirable output power level of the up-linktransmitter so as not to radiate excessive power yet guarantee adequatelevels.

Transponder/Quadrature Receiver. Application of a TV color "chip"integrated circuit 81 of FIG. 12c to T-NET transponders will beexplained in the following discussion to illustrate a practicaleconomical design for detecting the alternative downlink quadraturemodulation (FIG. 9c) but it should be emphasized that the essentialfeature of interest in this discussion has to do with the fact that thislow-cost chip can be employed to demodulate both quadrature terms of theT-NET downlink subcarrier because it appears like the TV quadraturemodulated chroma signal.

A typical TV integrated circuit 81 includes an IF preamplifier 83 and IFamplifier 85 and detector 89 that are relatively conventional in designand include provision for automatic frequency tuning (AFT) circuit 87and automatic gain control (AGC) circuit 91. It also includes horizontaland vertical sync separation and detection circuits 93 and 95. It waspointed out earlier in this specification that in U.S. color televisionsystems the television signal is coded and transmitted as threecomponents: a monochrome luminence component "M" and two colorcomponents "I" and "Q" that are superimposed on a chroma subcarrierhaving a fixed precise frequency of 3.579545 MHz. That chroma subcarrieris quadrature modulated with the I and Q color signals essentially inthe same manner which can be employed for the alternative modulation ofthe T-NET gated subcarriers (FIG. 9c).

In the case of color television transmissions, a brief "chroma burst"synchronizing signal is transmitted by the television transmitter (seeFIG. 8) on the "back porch" of each horizontal sync pulse and itspurpose is to phase-lock voltage controlled oscillator 96 (FIG. 12c)with all television receivers. Therefore phase-locked oscillator 96 canbe used as a continuous phase reference to demodulate the I and Qcomponents of the transmitted color TV signal. In color TV receivers thechroma burst detector 103 accomplishes that synchronizing process byusing a time gate derived from H-sync detector 93 to gate out theapproximately 8 cycles of chroma burst; those 8 cycles are applied tophase-lock loop circuit 101 which controls VCO 96 and thereby keeps itprecisely in phase with the 8-cycle chroma burst. A precisely tunedquartz crystal at 3.579545 MHz is connected to terminal 105 and thiscauses phase-lock loop 101 and VCO 96 to remain precisely in phase withthe chroma burst oscillation even after the burst ceases. VCO 96 ineffect "coasts" during the time interval between chroma bursts withnegligible drift.

The manner in which the color TV integrated circuit 81 can be adapted sothat it can instead detect down-link quadrature modulated subcarrierdigital signals of the instant invention will now be described. Theobjective is to use burst detector 103, the I and Q detector 99,phase-lock loop 101 and VCO 96 for this purpose. The subcarrierfrequency synthesizer 98 previously described in regard to FIG. 12B isnow connected in place of the chroma oscillator quartz crystal at input105. It was pointed out earlier that the bandpass of the transponderreceiver is tuned so as to pass only the thirty-two T-NET subcarriersand the Host TV carrier frequency. Thus it does not pass the 3.579545MHz chroma burst or the chroma subcarrier. Consequently the chroma burstdetector, being gated to operate only during specified portions of thehorizontal blanking interval, will see instead a composite of many T-NETdata subcarrier frequencies, depending upon which subcarriers are beingemployed for down-link data transmission. Since frequency synthesizer 98is tuned to a specific subcarrier and is injecting signal into thephase-lock loop 101, it and VCO 96 can be forced to lock on to only thatspecific down-link subcarrier frequency. Consequently the referencefrequency injected into I and Q detector 99 by VCO 96 causes detector 99to demodulate the in-phase and quadrature phase (I and Q) digital datacomponents of that specific subcarrier only.

Consequently the readily available and inexpensive "color" televisionintegrated circuit 81 can be used to demodulate any one of the manyquadrature modulated data subcarriers used in the instant invention. Theoutput of detector 99 is connected to integrate and dump circuit 109where a synchronizing signal based on the horizontal sync signal from 93is used to accurately gate-out and optimumly detect the T-NET digitaldata of the subcarrier which has been selected. The very powerful HostTV carrier component and sync signals are consequently used toeffectively "carry" and thereby enhance T-NET transmission reliability;i.e. the synergetic modulation advantage.

The transponder microprocessor 36 shown in FIG. 3 serves the purpose ofcoordinating the timing of the transponder's radio sections as well asbuffer storing and relaying messages between it and the companion device(e.g. personal computer). It also performs certain housekeepingfunctions such as recognizing which incoming messages it is to detectand pass on. It also helps the network control center coordinate overallT-NET system traffic by dynamically shifting to subchannel frequenciesassigned to it to transmit and receive on, either as directed by thenetwork control center or as assigned by the companion device. Themicrorpocessor is conventional in its design and its programming isrelatively straight forward.

Transponder/Duplexer. The transponder duplexer 32 (FIG. 12) permitssharing of the subscriber's existing TV antenna with the existingtelevision receiver, the transponder's receiver section, and itstransmitter section. Its principal job is to isolate the transponderreceiver and television rec iver from the transmitter section so thatthey will not be damaged while it transmits. FIG. 13 shows one possibleduplexer design for isolating these receivers from the transmitter. Thesubscriber's TV antenna is connected to the transponder receiver andtelevision receiver through a one-quarter wave length coaxial cable 114which has at its output end a diode switch 116 that is controlledthrough a radio frequency choke 118 by microprocessor 36. When thetransponder is required to transmit a data pulse, diode 116 is switchedto a low impedance state by microprocessor 36 and this in effect shortcircuits the output end of coaxial cable 114 and causes a reflected opencircuit impedance at its input end (the left side in FIG. 13).Consequently the RF pulses generated by transmitter 38 see an opencircuit at the input to coax 114 and the signals are consequently routedon to the subscriber TV antenna and radiated outward.

On the other hand, when transmitter 38 is not transmitting, which ismost of the time, signals coming into subscriber TV antenna 12 passthrough coax 114 and into signal splitter 120 where they are routed bothto the existing television receiver so that it may receive conventionaltelevision programs and also to receiver 34, which is part of thetransponder. Under these receiving conditions transmitter 38 representsan open circuit and it rejects the incoming received signals.

Transponder duplexer 32 of FIG. 13 is only one of several methods whichcan be used. For example, devices referred to as microwave circulatorscomprise a three port passive network which can accomplish a comparablefunction and have the additional advantage of being broadband.

We shall now describe the major components of a radio central office andwill emphasize the unique and novel circuits which have been devised topractice the instant invention.

Radio Central Office (RCO). FIG. 14 is an overall block diagram of atypical radio central office. Antennas 28 represent one of a pluralityof directional antennas, each connected exclusively to a separate sectorreceiver 122. Each sector receiver 122 covers the entire 6 MHz TVchannel which has been assigned to the T-NET system. For example, tenantennas 28, each having an 18-degree beamwidth, connected to tenreceivers 122 will provide a 180-degree coverage. If each angular sectoruses sixteen of thirty-two subchannels in a system where odd numberedsubchannels are used on one sector, even numbered used on the adjacentsector, and the odd number again used on the next adjacent sector . . .etc., then the arrangement would be as shown in FIG. 14. In that case,each sector receiver 122 would require sixteen filters 124 and these areshown as divided into two banks of eight subchannel filters each; thebank of filters shown in the top row of FIG. 14 cover subchannels 9through 16 and the lower row of eight filters cover channels 1 through8. Each of the subchannel filters 124 is connected to its separatedigital interface circuit card 126 and they are numbered in acorresponding manner.

In one preferred embodiment eight of these subchannel filters andassociated digital interface circuits can be controlled from onesingle-board computer 128 and this is the reason that FIG. 14 shows twogroups of eight subchannel filter/digital interface circuits connectedto a "B" single-board computer 128, and 8 more subchannel filter/digitalinterface circuits connected to a "A" single-board computer 128. Thussector #1 receiver feeds sixteen subchannel filters 124, sixteen digitalinterface circuit cards 126, and two single-board computers 128. If aT-NET system had ten sector antennas and ten associated sectorreceivers, there would be a total of three hundred sixty subchannelfilters 124 and digital interface cards 126 and twenty single-boardcomputers 128.

Protocol computer 130 (FIG. 14) collects the data from all single-boardcomputers 128 and reformats and buffer stores it as necessary and thentransmits it through trunkline 26 to network control center 2 (shown inFIG. 1). That trunkline may be any one of several commonly used links,such as a microwave link illustrated FIG. 14.

Display and I/0 133 shown in FIG. 14 is a computer monitor andinput/output (I/0) device which may be employed to input the rangeaddress of the many subscribers who sign up for this communicationservice. It may also be used for overhead functions such as monitoringthe activity of specific digital interface circuits 126, single boardcomputers 128, or for trouble shooting purposes.

RCO/Sector Receiver FIG. 15 shows the block diagram of a typical sectorreceiver subsystem Antenna 28 detects up-link signals from subscribertransponders and connects them to bandpass filter 134 and adjacentchannel rejection filter 136. These filters suppress most of the videocomponents of the Host television signal and other interference to thelevel where they will not overwhelm the subscriber signals. It should bepointed out that typical television transmitters have an effectiveradiated power (ERP) ranging from 25,000 to two million watts or moreand are consequently much more powerful than the subscriber signals.Incoming signals are further amplified in 138 and down converted infirst mixer 140. Intermediate frequency (IF) filters 142 and amplifier144 have approximately 6.0 MHz bandwidth and provide a sharp attenuationof all signals lying outside its bandwidth.

The output of amplifier 144 is connected to a second down convertermixer 146 and this is followed by a second IF filter 148 and amplifier150. The second IF is 21 MHz and it also has 6.0 MHz bandwidth. All thethirty-two subchannels of a T-NET system are emcompassed within thisbandwidth. The output of sector receiver 122 is connected in parallel toa bank of filters 24; one filter is required for each of the thirty-twosubcarriers used in the sector. Included within these bandpass filtersis a detector so that the output of each filter is the subcarrierbaseband with analog data, i.e. it is the sum of all digital pulsestransmitted by transponders in the sector. It has already been notedthat each of the bandpass filter/detector assemblies for a sector couldconsist of sixteen subchannels so one sector would only operate oneither odd channels or even channels in order to provide frequencyre-use from sector to sector.

RCO/Digital Interface. Each of the subchannel filters 124 has connectedto its output a digital interface circuit card and a block diagram ofthat card is shown in FIG. 16. The purpose of the digital interface cardis to create range gates at the proper time delay representing thedistance to each of the many subscribers operating on that subchannel soas to detect pulse from them and forward the data to its companionsingle board computer 128 (FIG. 14). We shall now explain the novelaspects of the block diagram in FIG. 16. All circuits of digitalinterface card 126 are interconnected to other assemblies through astandard multi-bus 156. For example, it has already been pointed outthat there will be eight digital interface cards for each single boardcomputer 128 and these will be interconnected through multibus 156.Display and I/0 device 133 may also be connected through that multibus.

In one mode of operation the display and I/0 device 133 (FIG. 14) isused to input the range address of a new subscriber based on known orcomputed range to that subscriber and this information will be "logged"into subscriber range address memory 160 through channel address decoder158 and data bus 157. At the same time, the strength of that subscribersignal will be either measured or estimated and that information will beinput to subscriber amplitude signature memor 166 in like manner. Thusthe range and amplitude of each subscriber transponder will be held inmemory 160 and 166 respectively. The analog pulses from subchannelfilter 124 representing incoming data from each transponder is connectedto integrate and dump analog circuit 164.

The operation of digital interface card 126 is repetitive and triggeredinto operation by the vertical and horizontal sync pulses of the Host TVstation as detected by a separate receiver using antenna 27 (FIG. 3).These sync pulses are connected to address generators 174 and 176. Uponthis triggering, subscriber address counter 176 begins to count upwardto generate addresses in a series of steps, each step being proportionalto the distance the H sync pulses have propagated outward from thetelevision station as it sweeps across the countryside. In other wordssubscriber address counter 176 will have developed a count which isequal to the distance from the television station to the instantposition of the propagating Host TV horizontal blanking interval.

Subscriber address counter 176 is connected to the subscriber rangeaddress memory 160 and amplitude memory 166 and if those memorylocations hold a subscriber, that fact is caused to trigger gategenerator 162 and A/D 168. Thus a comparison is constantly being made byrange address gate generator 162 to see if any subscriber lives at thecurrently developed address count; if there is, range gate addressgenerator 162 generates a range gate which enables integrate and dumpanalog circuit 164 to accept and integrate the pulse from that specifictransponder at that specific range address. At the end of a fivemicrosecond integrate period the pulse from that transponder isinstantaneously compared in comparator 170 against a threshold levelwhich has been established by digital-to-analog converter 168, which inturn is dependent upon the expected strength of that subscriber. Basedon the result of analog comparator 170 a determination is made as towhether there is a logic "1" transmission or a logic "0" (notransmission) from the transponder at that specific range address. Itcan be appreciated that these comparisons are done on a microsecond bymicrosecond basis and in accordance with a prearranged scheduledepending upon which subscribers are logged in memory and what theirdistance is from the Host television transmitter. It can be furtherappreciated that pulses from many subscribers are all time interleavedand must be sorted out; that is the job of double buffer demux 172.

The double buffer demux 172 circuit has connected to it an addressgenerator (counter) 174 which is triggered into operation by the Host TVvertical and horizontal sync pulses and it first generates a coarse timedivision component of subscriber address (i.e. the specific H-syncpulses that the transponder has been assigned to operate on) and asecond fine time division address based on the range to each subscriber.Demux 172 also has as input the output of comparator 170 which comprisesdigital pulses from each of the many subscribers assigned to thatsubchannel. The double buffer demux 172 sorts out these time interleavedtransponder pulses and reorganizes them into data files in which thedata from each transponder is grouped together with the tranponder'saddress and placed into a buffer storage location. That buffer storageis periodically "dumped" into the multibus for transfer to thesingle-board computer 128. The single-boar computer 128 also receiveslike-data from seven other digital interface cards as shown in FIG. 14.The single-board computer groups all of this information and forwards itto protocol computer 130 where it is properly queued with the output ofmany other single board computers and forwarded over a trunklink 26 tothe notwork control center as previously explained.

Cable TV Design. We shall now describe the application of the T-NETsystem to cable TV (CATV). CATV design engineers have found that aproblem exists when many subscribers are connected to a coaxial cablefor reverse transmissions from subscribers to a central location. Thisproblem is due to the fact that each of the subscribers contributes afinite amount of noise and the cumulative effect of all of this noiseseriously reduces each of their signal-to-noise ratios, and perhaps alsothe down-link TV program. This problem is self defeating in thatincreasing the power of each subscriber does not offer any solutionbecause that also increases their cumulative noise. The instantinvention solves this problem because the T-NET transponders onlytransmit pulses and these pulses only exist at time intervals which aredistinct and separate for each transponder. Therefore their cumulativeeffect is negligible.

FIG. 17 illustrates a cable TV application of the T-NET system. Atransponder 14 operates substantially in the same way as described inthe preceding sections of this specification. TV and data signals fromthe coaxial cable are connected through transponder 14 and CATV tuner182 to TV receiver 112. The data output of transponder 14 is connecteddirectly to TV receiver 112 to provide interactive television operation.The talk-back feature could be through hand-held remote controller 18.

Transponder 14 (FIG. 17) detects the down-link data and sync pulses ofthe Host TV station 10 signal injected at CATV "Head-End" 177 andreceives in the same manner already described. The transponder 14up-link reply pulses are sent through coaxial cable 183 and arecollected at a multiplexed repeater 180 which could be located withinthe existing cable TV amplifier boxes 181 which are typically spaced atintervals less than one mile. The multiplexed repeater 180 can bedesigned to function essentially as a multiplexed pulse transponder(somewhat like 14) so as to collect and retransmit up-link signalsdetected by 179. These are appropriately synchronized to the local TVsignal and radiated through antenna 184. Usually there would not be morethan a few dozen transponders 14 connected between cable TV amplifierboxes. Their "range address" is determined by the Host TV-to-subscriberindirect distance which is the combined cable and over-the-air effectivedistance. The design of these multiplex transponders (repeaters) will beobvious to those skilled in the art after studying the several drawingsand discussion presented in this specification and duly observing theH-sync requirements on the cable TV signal because it is off-set fromthe over-the-air TV signal H-sync.

The radio central office detects and process these semi CATV signals inmuch the same manner as it already processes transponder replies frompurely over-the-air subscribers. The output of multiplex repeater 180would in fact appear like interleaved pulses from several dozentransponders. The fact that these particular CATV trasnponders operatepartly through a coaxial cable would be transparent to the radio centraloffice. A similar arrangement could be used in large office buildingswhich use a coaxial cable and common antenna.

Vehicle Location Design. We shall now discuss a vehicle locationapplication of the T-NET system (FIG. 18). It has been pointed out thatthe range to each fixed location subscriber represents its "rangeaddress" and this information is kept in memory in each radio centraloffice. On the other hand, if the transponder is a portable device or ina vehicle, then its range will initially be unknown. Specificsubchannels can be dedicated to operate only with such movingtransponders

When a precise determination of the transponder location is desired, theT-NET system can be desi9ned to provide for detection of up-link si9nalsby at least two central receivers labeled #1 and #2 in FIG. 18. Theposition and distance between these two central receivers will beprecisely surveyed to establish a fixed baseline from which the positionof each transponder can be accurately computed based on precisemeasurement of the range from the transponder to central receivers #1and #2. Such computations are well known and commonly employed in radionavigation. FiG. 18 shows such an operation in which vehicle 186 detectsHost television data and sync signals through mobile antenna 188 andconnects those signals to RF modem 14. The demodulated signals frommodem 14 are connected to vocoder 194 and to computer/monitor 196.

Up-link data pulses from modem 14 are detected by both central receiver#1 and #2 through their antennas 128. Since the range to the transponderis unknown initially, a series of sequential range gates is generated bycentral receiver #1 and #2 and each of these gates is examined insequence to find where pulses are being received When this isdetermined, a pair of range gates, called an "early" and "late" gate inradar terminology, are positioned around the received pulses so as totrack it as the vehicle moves. Such pulse acquisition and trackingtechniques are well known in the art of radar circuit design. The rangeinformation which is thus measured is communicated from central receiver#2 to central receiver #1 where a navigation computation algorithm canbe installed in a conventional computer to solve the triangulationproblem to precisely locate and track vehicle 186. That positioninformation can be forwarded to the network control center and/or to aHost such as a vehicle dispatcher. Indeed, the navigation computer couldbe installed at the Host computer, if that were more convenient.

The vocoder 194 (FIG. 18) is intended to be a voice-to-digital anddigital-to-voice converter which takes the output of microphone 190 anddigitizes it so that it may be sent through RF modem 14. Likewise, thedigitized output of modem 14 can be converted to voice signals andtransmitted through speaker 192. This provides a means of verbalcommunication through RF modem 14. If RF modem 14 operates at 1200 baud,then it is too slow for direct digitized voice transmissions, however,microprocessor 36 in RF modem 14 can buffer-store and thus time-stretchand compress the 1200 baud digitized voice information in such a manneras to make it intelligible, although it may not permit effectivereal-time dialog between two speakers because of the time delay. This isreferred to herein as "slow-voice" or "voice messaging". On the otherhand, RF modem 14 could be designed to transmit at a rate up to 15,734baud and this is sufficiently fast to provide real-time voicetransmissions through a vocoder 194, if this were desirable.

Down-Link Co-channel Modulator. We shall now describe the manner inwhich digital data may be sent co-channel simultaneously with a regulartelevision program without interfering with it. This is referred to asco-channel multiplexed data and video in FIG. 19. It has already beenexplained that the object of this technique is to superimpose digitaldata onto the regular television video on each of the 525 lines of a TVframe and then, on the succeeding frame, to superimpose the same digitaldata, but inverted, so that at each corresonding element (pixtel) of theTV picture the data is first added and then subtracted so as to becomeinvisible. It was pointed out that this could be done throughout theentire TV picture at a sacrifice in picture quality in motion segments,or it can be restrained to only those portions of the picture thatconvey fixed scenes.

FIG. 19 shows the block diagram of a system of this invention forsending digital data only in the fixed scen portion of TV pictures. Thedigital data to be transmitted is buffer-stored in digital memory 200and read out from that storage device at prescribed times and sentthrough amplifier 202 to a split channel to provide data and inverteddata into switch 206. One frame of 525 lines of regular TV video isconnected to A/D converter 208 where it is digitized and stored inmemory 210. The video from a following frame of 525 lines is comparedpixtel-by-pixtel in 214 against the corresponding information storedfrom the previous frame in order to detect where differences (motion)exist. Where the second and first frame picture elements are identical,it is assumed to represent a fixed scene segment and in that event anenable command is sent by 214 to buffer store 200 and digital data isoutput from that buffer and is connected to summing circuit 212. Switch206 use TV V-sync to reverse its position every TV frame so that thedata is first added in one frame and subtracted in a subsequent frame.

In effect comparator 214 is constantly comparing the output of thecurrent TV frame against the previous TV frame in order to find fixedscene locations so that it can transmit data in those segments. Theoutput of 214, being the current frame, is D/A converted in 216 so as torestore the original analog video which is sent to summing circuit 212where the data is added to it. The output of summing circuit 212represents video plus and minus data and it is sent to the regular TVtransmitter for transmission to TV viewers and to transpondersspecifically designed to detect the data portion of the TV signal. Suchdata receivers could operate essentially like present day Teletextreceivers but it would include circuits which take advantage of theredundant transmission (i.e. Data+DATA) for more reliable detection.

T-NET/Cellular Radio Integration. The vehicle location capabilities ofthe T-NET system can be used to advantage to initiate and coordinate thehand-off of cellular radio telephone subscribers. FIG. 20 illustratessuch an application. Two T-NET radio central offices labeled RCO #1 andRCO #2 are located with respect to cellular system 218 so that they maydetermine the position of any vehicle 222. Although the hexagon shape220 defining the various cellular limits are useful in populardescriptions of the idea that cellular radio is partitioned intoindividual cells, it is clear in actual practice the geometry of anyspecific cell may take any arbitrary shape such as 224. This is due tothe fact that the only information available to the cellular radiosystem as to the position of vehicle 222 is its signal strength. Signalstrength is not a reliable indicator of vehicle position because itvaries from time to time and because of local reflections frombuildings, other vehicles and for other physical reasons. On the otherhand, using a relatively simple radio survey, a geometric area such as224 can be found where reliable transmission with all vehicles in thatarea can be established and maintained. Many areas such as 224 can befound so that complete coverage of the entire service area 218 can beassured. In a practical world those areas 224 constitute the real cells.

Consequently if one has independent means such as a T-NET system todetermine in which cell 224 a subscriber is located, then the problem ofhanding-off vehicle 222 as it moves from cell-to-cell becomes arelatively simple computer function. This is also a relatively simpleprocess for the T-NET vehicle location mode to accomplish. It would alsooccupy very little of its traffic capabity. Furthermore, a specializedT-NET transponder could be designed and built for this function alone toreduce its cost and increases its reliability in this operating mode.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention

I claim:
 1. A bidirectional wireless digital communication systemcomprisinga broadcast station for transmitting a video signal at leastincluding horizontal blanking intervals and associated horizontal syncsignals on a video carrier; broadcast means for controllablytransmitting downlink digital data signals: a plurality of subscriberreceiver-transmitters, each subscriber receiver-transmitter having asubscriber receiving means for receiving said video signal and detectingsaid horizontal sync signals, and for receiving and detecting saiddownlink digital data signals, each subscriber receiver-transmitter alsohaving a subscriber transmitting means coupled to said subscriberreceiving means for transmitting uplink digital data signals only duringat least some of the horizontal blanking intervals of the received videosignal; and at least one central receiver, each said central receiverbeing a means for receiving and detecting said uplink digital datasignals transmitted by each subscriber transmitting means.
 2. The systemof claim 1 wherein said broadcast means includes means for modulatingsaid downlink digital data on a downlink data carrier having asubstantially fixed relationship to and different from said videocarrier prior to transmitting said downlink digital data signals.
 3. Thesystem of claim 2 wherein said video carrier is the video carrier of afirst conventional television channel.
 4. The system of claim 3 whereinsaid downlink data carrier is in the frequency band assigned to aconventional television channel adjacent said first conventionaltelevision channel.
 5. The system of claim 4 wherein said broadcastmeans includes means for modulating downlink digital data on a pluralityof downlink data carriers, each of said downlink data carriers beingdifferent from the others and within the frequency band assigned to aconventional television channel adjacent said first conventionaltelevision channel.
 6. The system of claim 5 wherein said downlink datacarriers are sufficiently separated in frequency whereby the modulateddownlink digital data on each downlink data carrier may be received anduniquely detected in a subscriber receiver, at least in part byfrequency discrimination.
 7. The system of claim 6 wherein the modulateddownlink digital data is quadrature amplitude modulated on said downlinkdata carriers.
 8. The system of claim 5 wherein multiple bits ofdownlink digital data are transmitted on each downlink data carrierduring at least some of the horizontal blanking intervals, and whereinthe baud rate during the horizontal blanking intervals is a harmonic ofthe repetition rate of the horizontal sync signals, whereby bit clocksmay be synthesized from the horizontal sync signals.
 9. The system ofclaim 1 wherein each subscriber transmitting means includes means formodulating uplink digital data on an uplink data carrier having asubstantially fixed relationship to and different from said videocarrier prior to transmitting uplink digital data signals.
 10. Thesystem of claim 9 wherein said video carrier is the video carrier of afirst conventional television channel.
 11. The system of claim 10wherein said uplink data carrier is in the frequency band assigned to aconventional television channel adjacent said first conventionaltelevision channel and wherein said modulated uplink digital data istransmitted by a subscriber transmitting means only in at least some ofthe horizontal blanking intervals of said video signals as received bythe respective said subscriber receiving means.
 12. The system of claim1 wherein said video carrier is the video carrier of a firstconventional television channel and wherein at least some of saidsubscriber transmitting means include means for modulating uplinkdigital data on a different one of a plurality of uplink data carriers,each of said plurality of uplink data carriers being a differentfrequency from the others and within the frequency band assigned to aconventional television channel adjacent said first conventionaltelevision channel.
 13. The system of claim 12 wherein said uplink datacarriers are sufficiently separated in frequency whereby the modulateduplink digital data on each uplink data carrier may be received anduniquely detected in said central receiver, at least in part, byfrequency discrimination.
 14. The system of claim 13 wherein themodulated uplink digital data is quadrature amplitude modulated by saiduplink data carriers.
 15. The system of claim 12 wherein multiple bitsof uplink digital data are transmitted on each uplink data carrierduring at least some of the horizontal blanking intervals, and whereinthe baud rate during the horizontal blanking intervals is a harmonic ofthe repetition rate of the horizontal sync signals, whereby bit clocksmay be synthesized from the horizontal sync signals.
 16. The system ofclaim 12 wherein said broadcast means includes means for modulatingdownlink digital data on a plurality of downlink data carriers, each ofsaid downlink data carriers being different from the others and withinthe frequency band assigned to a conventional television channeladjacent said first conventional television channel.
 17. The system ofclaim 16 wherein at least some of said downlink data carriers and atleast som of said uplink data carriers have the same carrier frequency.18. The system of claim 17 wherein said broadcast means transmitsdownlink digital data on a downlink data carrier during a horizontalblanking interval in which a subscriber transmitting means transmitsuplink digital data on an uplink data carrier of the same frequency asthe last named downlink data carrier, said downlink and uplink datasignals being distinguishable by said at least one central receiver, atleast in part, by the difference in arrival times of the transmittedsignals.
 19. The system of claim 1 wherein said broadcast means isphysically independent of said television broadcast station, andincludes receiving means for receiving said video signal from saidtelevision broadcast station and detecting said horizontal sync signalsto determine when said broadcast means is to transmit digital datasignals.
 20. The system of claim 1 wherein said broadcast means iscoupled to said television broadcast station, whereby said televisionbroadcast station broadcasts through its antenna a video signal at leastincluding horizontal blanking intervals and associated horizontal syncsignals, at least some of said blanking intervals including a digitaldata signal.
 21. The system of claim 1 wherein said at least one centralreceiver is positioned approximately between said televisio broadcaststation and said plurality of subscriber receiver-transmitters.
 22. Thesystem of claim 1 wherein said television broadcast station transmits avideo signal comprising regular television programming, said broadcastmeans being coupled to said television broadcast station so that saidtelevision broadcast station broadcasts through its antenna regulartelevision programming and downlink digital data, said downlink digitaldata being transmitted as video information by adding said downlinkdigital data to at least part of the video signal during one video frameand subtracted from the corresponding part of the video signal duringthe next video frame.
 23. The system of claim 22 wherein the parts ofthe video signal to which downlink digital data are added and to whichdownlink digital data are subtracted are portions of the video signalwhich are substantially unchanged during the two successive videoframes.
 24. The system of claim 1 wherein said television broadcaststation antenna is substantially colocated with the broadcast antenna ofat least one more television channel, and wherein the video signals ofeach of said substantially colocated channels are synchronized so thatthe horizontal blanking signals thereof coincide in time.
 25. A systemin accordance with claim 1 for two way paging wherein said subscriberreceiver-transmitters are mobile.
 26. A bidirectional wireless digitalcommunication system comprisinga television broadcast station fortransmitting ordinary television programming including vertical sync andhorizontal sync signals and associated blanking intervals on apreassigned television channel and associated video carrier; broadcastmeans for controllably transmitting downlink digital data signals; aplurality of subscriber receiver-transmitters distributed about an areawithin the broadcast range of said television broadcast station and saidbroadcast means, each subscriber receiver-transmitter having asubscriber receiving means for receiving said video signal and detectingsync signals, and for receiving and detecting said downlink digital datasignals, each subscriber receiver transmitter having modulating meansfor modulating uplink digital data signals on an uplink carrier of afrequency within the frequency band of a television channel adjacentsaid preassigned television channel, each subscriber transmitting meanscoupled to said subscriber receiving means for transmitting themodulated uplink digital data signals during at least some of theblanking intervals of the received video signal; and a plurality ofcentral receivers, each central receiver being located and having adirectional antenna to predominately receive the modulated uplinkdigital data signals from a respective subarea within said area, eachsaid central receiver being a means for receiving and detecting saiduplink digital data signals transmitted by the respective subscribertransmitting means within the respective subarea.
 27. The system ofclaim 26 wherein each subscriber transmitting means includes means formodulating uplink digital data on an uplink data carrier having asubstantially fixed relationship to and different from said videocarrier prior to transmitting uplink digital data signals.
 28. Thesystem of claim 27 wherein said uplink data carrier is in the frequencyband assigned to a conventional television channel adjacent said firstconventional television channel and wherein said modulated uplinkdigital data is transmitted by a subscriber transmitting means only inat least some of the blanking intervals of said ordinary televisionprogramming as received by the respective said subscriber receivingmeans.
 29. The system of claim 26 wherein at least some of saidsubscriber transmitting means include means for modulating uplinkdigital data on a different one of a plurality of uplink data carriers,each of said plurality of uplink data carriers being a differentfrequency from the others and within the frequency band assigned to aconventional television channel adjacent said preassigned televisionchannel.
 30. The system of claim 29 wherein said uplink data carriersare sufficiently separated in frequency whereby the modulated uplinkdigital data on each uplink data carrier may be received and uniquelydetected in said central receiver, at least in part, by frequencydiscrimination.
 31. The system of claim 30 wherein the modulated uplinkdigital data is quadrature modulated by said uplink data carriers. 32.The system of claim 30 wherein multiple bits of uplink digital data aretransmitted on each uplink data carrier during at least some of thehorizontal blanking intervals, and wherein the baud rate during thehorizontal blanking intervals is a harmonic of the repetition rate ofthe horizontal sync signals, whereby bit clocks may be synthesized fromthe horizontal sync signals.
 33. The system of claim 30 wherein saidbroadcast means includes means for modulating downlink digital data on aplurality of downlink data carriers, each of said downlink data carriersbeing different from the others and within the frequency band assignedto a conventional television channel adjacent said first conventionaltelevision channel.
 34. The system of claim 33 wherein at least some ofsaid downlink data carriers and at least some of said uplink datacarriers have the same carrier frequency.
 35. The system of claim 34wherein said broadcast means transmits downline digital data on adownlink data carrier during a horizontal blanking interval in which asubscriber transmitting means transmits uplink digital data on an uplinkdata carrier of the same frequency as the last named downlink datacarrier, said downlink and uplink data signals being distinguishable bysaid at least one central receiver, at least in part, by the differencein arrival times of the time signals.
 36. A system in accordance withclaim 1 for two way paging wherein said subscriber receiver-transmittersare mobile.
 37. A system in accordance with claim 26 for interactivetelevision wherein said subscriber receiver-transmitters are coupled toa television receiver for displaying on a display the ordinarytelevision programming and for also displaying downlink information,said subscriber receiver-transmitters including means for user entry ofuplink digital data at the respective subscriber receiver-transmitter.38. The system of claim 26 wherein said subscriber receiving means andsaid subscriber transmitting means both share a television antenna withat least one ordinary television receiver.
 39. A method of informationcommunication within an area utilizing the frequency band of atelevision channel adjacent that of a television channel serving thesame area and broadcasting ordinary television programming, includinghorizontal and vertical sync signals and associated blanking intervals,comprising the steps of:(a) modulating information on a carrier withinthe frequency band of the television channel adjacent that of thetelevision channel broadcasting ordinary television programming; (b)transmitting the modulated information only during the horizontalblanking intervals of the ordinary television programming; (c) receivingthe transmitted modulated information at a distant location anddetecting the information therein.
 40. The method of claim 39 wherein instep (c), the information is detected in part by also detecting thehorizontal sync signals of the ordinary television programming andreferencing the detection thereto.
 41. The method of claim 40 whereinthe detection of step (c) is referenced to both the horizontal syncsignals of the ordinary television programming and the known distancebetween the position of transmitting of step (b) and the position ofreceiving of step (c).
 42. The method of claim 39 wherein instep (b) thetransmission is from the same antenna as the television channelbroadcasting ordinary programming material.
 43. The method of claim 39wherein in step (b) the transmission is from a different antenna as thetelevision channel broadcasting ordinary programming material.
 44. Themethod of claim 43 wherein is step (c), the transmitted modulatedinformation is received using a directional antenna locatedapproximately between the location of broadcast of the ordinarytelevision programming and the location of the transmission of step (b).45. A method of communicating information from a plurality of remotelocations to a central location served by a television stationbroadcasting ordinary television programming in the frequency band of afirst preassigned television channel, the television programmingincluding horizontal and vertical sync signals and associated blankingintervals, comprising the steps of:(i) at each remote location;(a)receiving the ordinary television programming and detecting thehorizontal sync signals thereof, (b) modulating the information to becommunicated from each remote location onto a carrier having a frequencyin the frequency band of a preassigned television channel adjacent thefrequency band of the first preassigned television channel, (c)transmitting the modulated information during at least some of thehorizontal blanking intervals of the received television programming,and (ii) at at least one central location(d) receiving the transmissionsof step (c) and detecting the information therein.
 46. The method ofclaim 45 wherein the detection of step (d) is referenced to thehorizontal syn signals of the ordinary television programming.
 47. Themethod of claim 46 wherein the information is in digital form, wherebythe carrier of step (b) is a data carrier.
 48. The method of claim 47wherein a plurality of data carriers are used, each having a frequencyin the frequency band of a preassigned television channel adjacent thefrequency band of the first preassigned television channel, each remotelocation modulating the data in step (b) on a specific one of the datacarriers assigned thereto.
 49. The method of claim 48 wherein said datacarriers are distributed throughout the frequency band of the respectivetelevision channel.
 50. The method of claim 48 wherein in step (b) themodulated information is transmitted only during specific horizontalblanking intervals as assigned to that remote location, whereby thetransmission from different remote locations may be during differenthorizontal blanking intervals to provide a plurality of data channels.51. The method of claim 50 wherein the specific horizontal blankingintervals used for transmission in each remote location are determinedwith reference to the reoccurence of the vertical sync pulse of theordinary television programming.
 52. The method of claim 51 wherein thedetection of step (d) is further referenced in time to the knowndistances of transmission, whereby data may be modulated on the samedata carrier at different remote locations in step (b) and transmittedduring the same horizontal blanking intervals in step (c) andunambiguously detected in step (d) by range gating at the detector,thereby providing additional data channels differing in range.
 53. Themethod of claim 52 wherein plurality of central locations are used forstep (d) and wherein the reception of step (d) is by way of adirectional antenna, whereby different central locations will receiveand detect digital data from different groups of remote locations,providing still additional data channels.
 54. The method of claim 53wherein said plurality of control locations are physically positionedapproximately between the television station antenna and the group ofremote locations served thereby.
 55. The method of claim 54 wherein instep (c) a plurality of data bits are transmitted from at least oneremote location during at least some of the horizontal blankingintervals, and wherein the bit rate is a harmonic of the repetition rateof the horizontal sync signals, whereby the bit clocks at the remotelocations and at the at least one central location may be synchronizedby the ordinary television programming horizontal sync signal.
 56. Themethod of claim 54 wherein the modulation of step (b) is a quadraturemodulation.
 57. The method of claim 45 wherein at each remote location,the transmission of step (c) is on a television antenna shared with atleast one ordinary television receiver.
 58. In an interactive televisionsystem, a method of communicating information from a plurality of remotereceiver locations, each connected to a cable television system, to acentral location, the cable television system receiving ordinarytelevision programming over the air and providing the same over a cablehaving amplifiers at various points along the cable, each serving aplurality of remote locations, comprising the steps of:(i) at eachremote location;(a) modulating the information to be communicated fromeach remote location onto a carrier having a frequency in the frequencyband of an unused cable television channel; (b) transmitting themodulated information on the cable only during at least some of thehorizontal blanking intervals of an adjacent cable channel carryingordinary television programming: (ii) adjacent each cable amplifier(c)detecting the horizontal sync signals of a television channelbroadcasting ordinary television programming over the air (d)transmitting the modulated information received on the cable over theair and in the frequency band of a television channel adjacent thechannel referred to in step (c) only during at least some of thehorizontal blanking intervals associated with the detected horizontalsync signals of the channel referred to in step (c); and (iii) at atleast one central location(e) receiving the transmissions of step (d)and detecting the information therein.
 59. A method of informationcommunication within an area utilizing the frequency band of atelevision channel adjacent to as well as that of a television channelserving the same area and broadcasting ordinary television programming,the programming including horizontal and vertical sync signals andassociated blanking intervals, comprising the steps of:(a) modulatinginformation on a plurality of subcarriers; (b) modulating the modulatedinformation of step (a) onto the carrier of the television channelbroadcasting the ordinary television programming, the subcarrierscausing the modulation of this step (b) to result in frequencycomponents within the frequency band of the television channel adjacentthat of the television channel broadcasting ordinary televisionprogramming; (c) broadcasting the modulated information only during thehorizontal blanking intervals of the ordinary television programming;(d) receiving the transmitted modulated information at a distantlocation and detecting the information therein.
 60. The method of claim59 wherein the modulation of step (b) also results in frequencycomponents within the frequency band of the television channelbroadcasting ordinary television programming.
 61. The method of claim 59wherein the modulation of step (b) is a single sideband modulation. 62.A method of determining the location of a mobile unit within an areautilizing the frequency band of a television channel adjacent to or thesame as that of a television channel serving the same area andbroadcasting ordinary television programming, including horizontal andvertical sync signals and associated blanking intervals, comprising thesteps of:(a) modulating identification information on a carrier withinthe frequency band of the television channel adjacent to or the same asthat of the television channel broadcasting ordinary televisionprogramming; (b) transmitting the modulated information only during thehorizontal blanking intervals of the ordinary television programming;(c) receiving the transmitted modulated information at the mobile unitand transponding the received information; and (d) receiving thetransponded informtion and the television programming at at least twodifferent central locations and determining the location of the mobileunit by differences in the transmission times of the received signals.63. The method of claim 62 wherein a video carrier including horizontaland vertical sync signals and associated blanking intervals is broadcastduring periods the ordinary television programming is not on the air.